Chief Superintendent Roberto Calinisan said the smoke bombs were contained in two canisters placed inside a bag which was found along the expressway at the boundary of Sto. Tomas and this town at about 2:45 p.m.
"Smoke bombs are used merely to camouflage or just to pinpoint landing areas for aircraft. Tear gas is much more harmful than a smoke bomb," he said.
"I am sure the canisters (containing the smoke bombs) were the handiwork of mere pranksters who were out to have fun and see the effect of their joke," he added.
Calinisan said police had to stop traffic along the south-bound lane of the expressway, causing a five-kilometer long traffic jam up to San Simon town, after they found the smoke bombs.
Meanwhile, Calinisan backtracked on his earlier claim that the region faces bombing threats from political opposition groups.
He said political opposition groups in Central Luzon pose no threat to peace and order. "They are merely exercising their democratic rights (during rallies and other protest activities calling for the resignation of President Estrada)," he said.
Calinisan added that the region is also not facing bombing threats from local Muslims. Authorities said the bomb explosions which rocked Metro Manila last Dec. 30 could have been the handiwork of the separatist Moro Islamic Liberation Front (MILF).
"I know the Muslims in Central Luzon. They are here for livelihood and not to resort to crimes," Calinisan said.
Calinisan noted that while there are yet no indications that communist groups would carry out bombings in Central Luzon, he said "the possibility is there."
"Communist groups are out to undermine the government, embarrass it, and create trouble," he said.
Last Friday, panic struck the Republic Central Colleges (RCC) in Angeles City after a caller warned that a bomb would explode in the campus. The call turned out to be a prank.
The leftist group Bagong Alyansang Makabayan (Bayan) has blamed the Estrada administration for the bombings in Metro Manila.